Integrated circuits (IC) are often fabricated with one or more semiconductor devices, which may include diodes, capacitors, and different varieties of transistors. These devices are generally fabricated by creating thin films of various materials, e.g. metals, semiconductors or insulators, upon a substrate or semiconductor wafer. Wafer and substrate are used interchangeably to refer to semiconductor structures during processing, and may include other layers that have been fabricated thereon. The physical characteristics and tightly controlled placement of films on a substrate will define the performance of the semiconductor device and its surrounding circuitry. Many semiconductor devices require a dielectric layer that must be reliable. Specifically, the dielectric layer must be essentially free from defects that cause shorting through the dielectric layer. Oxides and nitrides are used to form dielectric layers in semiconductor devices.
One process for forming metal oxide thin films on semiconductor wafers is chemical vapor deposition (“CVD”). CVD is used to form a thin film of a desired material from a reaction of vapor-phase chemicals containing the chemical constituents of the material. CVD processes operate by confining one or more semiconductor wafers in a reaction chamber. The chamber is filled with one or more gases that surround the wafer. The gases for the deposition of metal oxides includes a metal precursor and a reactant gas, e.g. water vapor, to be introduced into the chamber at the same time. Energy is supplied within the chamber and particularly to the reactant gases near the wafer surface. A typical energy is heat applied to the substrate. The energy activates the reactant gas chemistry to deposit a film from the gases onto the heated substrate. Such chemical vapor deposition of a solid onto a surface involves a heterogeneous surface reaction of the gaseous species that adsorb onto the surface. The rate of film growth and the quality of the film depend on the process conditions. Unfortunately, the metal precursor and the reactant gas also react during the gas phase remote from the substrate. Such a gas phase reaction produces contaminants and/or involve a significant quantity of precursor so that an insufficient amount is available for deposition on the substrate. As a result, the gas phase reaction may become dominant and the thin film coverage is poor. That is, pinholes may be formed in the resulting metal oxide layer. Moreover, using water (H2O) gas as the reactant gas results in impurities, such as hydroxide (OH), remaining in the resulting metal oxide layer.
Semiconductor fabrication continues to advance, requiring finer dimensional tolerances and control. Modern integrated circuit design has advanced to the point where line width may be 0.25 microns or less. As a result, repeatability and uniformity of processes and their results is becoming increasingly important. Generally, it is desired to have thin films deposited on the wafer to save space. Yet reducing the thickness of films can result in pinholes and in less mechanical strength.
Another development in the field of thin film technology for coating substrates is atomic layer deposition (ALD). A description of ALD is set forth in U.S. Pat. No. 5,879,459, which is herein incorporated by reference in its entirety. ALD operates by confining a wafer in a reaction chamber at a typical temperature of less than 300 degrees C. Precursor gas is pulsed into the chamber, wherein the pulsed precursor forms a monolayer on the substrate by chemisorption. The low temperature limits the bonding of the precursor to chemisorption, thus only a single layer, usually only one atom or molecule thick, is grown on the wafer. Each pulse is separated by a purge pulse which completely purges all of the precursor gas from the chamber before the next pulse of precursor gas begins. Each injection of precursor gas provides a new single atomic layer on the previously deposited layers to form a layer of film. Obviously, this significantly increases the time it takes to depose a layer having adequate thickness on the substrate. As a numerical example, ALD has a typical deposition rate of about 100 Å/min and CVD has a typical deposition rate of about 1000 Å/min. For at least this reason, ALD has not met with widespread commercial acceptance.
In light of the foregoing, there is a need for fabrication of thin films which are thinner and have a reduced number of defects.